I would look a little bit further into the fuel system. Some Leopards have the ability to feed engines from either tank. Valves for that can be hidden. I find it unlikely fuel would make its way uphill and across the hulls to get to the other tank. Having said that I know zero about the 4000 fuel system. This assumes the tanks are in the bottom of the hulls. If they are forward near bridge deck height it might be possible.
Edit, found a fuel system diagram for the 4000. It does not appear you can cross feed the engines or change which tank fuel is returned into. I would make sure all 3 fuel return lines are open. System looks simple. Strange the fuel is moving.